In hydrocarbon exploration and production, for a wellbore to be deemed successful, it is important for the operator to have knowledge of exactly how far the wellbore is from certain geological features of interest, either above or below the wellbore itself.
Due to geological and petrophysical complexities, relying purely on the measurements from conventional logging devices provides little or no quantitative estimates of the distance of the wellbore from features of interest. This can result in the wellbore exiting or missing the targets that have been determined for the wellbore. This problem is even more pronounced for smaller targets.
In recent years, there has been a substantial increase in the drilling of “horizontal” wells. Such wells often have much greater productivity than the more standard “vertical” wells. It is well known in the art that these “horizontal” wells are not necessarily horizontal but rather have boreholes which follow within the boundaries of a producing subsurface zone which deviates from horizontal to some degree.
In the process of drilling such a borehole, it becomes necessary to guide the drill bit so that the borehole does not leave the boundaries of the subsurface producing zone. A boundary of a producing zone may be established by various non-oil bearing formations or it may be established by such borders as the oil-water contact level in the same producing formation. In order to avoid these boundaries and stay within the producing formation, means have been developed in the prior art, with varying success, to detect and subsequently avoid the various boundary stratum.
Two methods for detecting a boundary stratum are illustrated, respectively, in U.S. Pat. Nos. 4,786,874 and 4,601,353. Each of these methods employs a directionally focused sensor. One method generally describes a directionally focused gamma ray tool and the other method describes a directionally focused resistivity tool. These tools show a change in sensor readings as a boundary stratum is approached. The drill string may then be rotated as necessary to determine the position of the boundary stratum by the variation in magnitude of the sensor readings. Once the position of the boundary stratum is known, the driller can orient the bit to drill away from the boundary stratum.
In some cases, while drilling through horizontal producing zones, the driller's main concern may be with the oil-water contact boundary stratum rather than other boundary stratum on the sides of or above the producing zone. The driller may wish to keep the borehole a certain distance above the oil-water contact level so as to maximize the productive life of the well. Also, the driller will probably not want to turn upwards unnecessarily. In such a case, the driller does not necessarily need a directionally focused sensor to tell him in which direction the boundary stratum is located because he already has reasonable certainty that the boundary stratum lays below the present borehole path. In fact, if the motor type drilling assembly is being used, due to the occasional necessity to change the direction of the bit, a tool with a directionally focused sensor may be focused in the wrong direction to indicate the approach of an oil-water contact boundary stratum and therefore be unreliable. Moreover, the need to reorient the tool may create undesirable drilling operations.
At one time, the prior art provided no effective or acceptable method for calculating the approximate angle or dip of an approaching boundary stratum, even though it was recognized that such information would generally be useful to the driller for various reasons. It might affect the degree of turn the driller wishes to achieve. The driller will generally desire to make the borehole as straight as possible and avoid making relatively sharp turns for such reasons as given above. Normally, the driller will want to make no more of a turn than is necessary to avoid the boundary stratum.
To address these needs, it has been proposed in the prior art to utilize methods and apparatuses capable of taking resistivity measurements at multiple or variable depths of investigation. Those of ordinary skill in the art will understand the term “depth of investigation” as applied to resistivity measurements to refer to measurements of formation resisitivity at multiple or variable radial distances from the longitudinal axis of the borehole. Numerous examples of such methods and apparatuses have been proposed in the prior art,
The use of a logging tool capable of taking multiple or variable depth of investigation resistivity measurements to adjust the direction of drilling to maintain a drill string within a region of interest, especially in the context of “horizontal” or “directional” drilling, is described in detail in U.S. Pat. No. 5,495,174 to Rao and Rodney, entitled “Method and Apparatus for Detecting Boundary Stratum and Adjusting the Direction of Drilling to Maintain the Drill String Within a Bed of Interest.” Resistivity sensing at multiple depths of investigation is also described in detail in U.S. Pat. No. 5,389,881 to Bittar and Rodney, entitled “Well Logging Method and Apparatus Involving Electromagnetic Wave Propagation Providing Variable Depth of Investigation by Combining Phase Angle and Amplitude Attenuation.”
Despite the technological advancements in the prior art, as exemplified by the referenced Rao et al. '174 patent and/or the Bittar et al. '881 patent, there continues to be a need for improvements in techniques for detecting the approach of boundary stratum, especially while drilling horizontal wells, which will result in greater reliability and dependability of operation. In particular, while the prior art includes examples of techniques useful for determining, to some degree of approximation, relative proximity of a borehole to a geophysical boundary, there have not been shown effective means or methods for quantifying the distance between a borehole and a geophysical boundary.